It is a well known dilemma in the field of semiconductor devices, that on the one hand a current flow should be distributed as uniformly as possible over the entire active region surface of a semiconductor device such as a light emitting diode (LED), while on the other hand, the current injecting electrode or contact should be made as small as possible, i.e. covering the smallest portion of the active region surface as possible, to avoid blocking or reflecting the light emitted from the active region and for reasons of costs and complexity. The same holds true for current injecting electrodes or contacts on the surface of, or internally within, semiconductor devices other than LEDs, and also applies to current collecting electrodes. In general, the electrode surface is any current carrying surface such as a contact surface that is connected to a bonding wire or the like.
In view of the above dilemma, certain problems arise. Namely, when using an electrode surface that is smaller than the associated active region surface, the current flow becomes "crowded", i.e. the current density is highest, directly under the electrode surface and diminishes laterally away from the electrode surface. Moreover, since the electrode surface acts as a reflecting screen for the emitted light (e.g. in the case of an LED), it is especially important that a good current density is provided to the areas of the active region surface laterally away from the electrode surface. Therefore, it is a well known problem to provide lateral current spreading between the electrode surface and the active region surface.
Several different structural arrangements have been proposed in the prior art for achieving the desired lateral current spreading. Published European Patent Specification 0,434,233 (Fletcher et al.) discloses one possible solution for LEDs, wherein a relatively thick transparent window layer having a lower electrical resistivity than the active layers is arranged between the electrode surface and the active region surface. Particularly, the transparent window layer has a band gap greater than the band gap of the active layers and a resistivity at least an order of magnitude less than that of the active layers. A metal contact or electrode layer is formed over a portion of the surface of the transparent window layer. This published reference discloses that thicker window layers tend to be desirable, and layer thicknesses in the range from 2 to 30 .mu.m are suitable, while thicknesses in the range from 5 to 15 .mu.m are preferred.
Published European Patent Application 0,551,001 (Fletcher et al.) similarly discloses the use of a thick transparent layer arranged over the active region of an LED. This reference discloses the quasi-conical current spreading effect of such a uniform, thick, transparent window layer. The window layer is purposely rather thick, to avoid or minimize the internal reflection of the light emitted from the active region. The thickness is determined as a function of the width of the window layer and the critical internal reflection angle.
The article "High-Efficiency InGaAlP Visible Light-Emitting Diodes" by H. Sugawara et al., published in Jpn. J. Appl. Phys. Vol. 31 (1992), pages 2446 to 2451 also discloses an LED structure including a current spreading layer having a thickness of 7 .mu.m (see page 2449). FIG. 4 of this article clearly shows the improvement in total light emission or light emission efficiency when using the GaAlAs current spreading layer as compared to a structure without such a current spreading layer.
The article "Two-Fold Efficiency Improvement in High Performance AlGaInP Light Emitting Diodes . . . " by K. Huang et al. in Appl. Phys. Lett. 61(9), Aug. 31, 1992, pages 1045 to 1047, also discloses a thick window layer for achieving current spreading, wherein the window layer thickness is particularly in the range from 9 to 63 .mu.m, and especially 45 .mu.m. Once again, the window layer thickness is defined in view of the light reflection effects.
A substantial disadvantage of all of the above mentioned conventional thick window layers is that the window layer has a substantially greater thickness than the active layers, which typically have thicknesses less than 1 .mu.m. This very great thickness of the window layer leads to a high material consumption for forming the window layer, and also requires a very long time for carrying out the epitaxial growth using conventional equipment. While it is possible to use more complicated and more expensive equipment for carrying out the epitaxial growth more quickly, it is still not possible to reduce the material consumption and costs, and the equipment costs are increased even further.
The article "Highly Reliable Operation of Indium Tin Oxide AlGaInP Orange Light-Emitting Diodes" by J. F. Lin et al., published in Electronics Letters, Oct. 13, 1994, Vol. 30, No. 21, pages 1793 to 1794 discloses a current spreading window layer made of indium tin oxide (ITO). The disclosed ITO current spreading layer is 600 .ANG. thick. Disadvantageously, the production costs and material costs of an LED incorporating such an ITO current spreading layer are even considerably higher than conventional LEDs or LEDs with the above mentioned thick window layers.